The present invention relates to pressure transducers and more specifically to pressure transducers employed in highly corrosive environments.
A differential pressure transducer as wherein the present invention is applicable is shown in U.S. Pat. No. 3,967,504 to Lloyd T. Akely and assigned to the common assignee of this application. FIG. 1 is a simplified drawing of that apparatus.
FIG. 1 shows a block diagram of a flowmeter installation utilized to measure the flow of process fluid in pipe 10. As used herein, process fluid refers to any fluid, liquid or gases, for which pressure or differential pressure, however generated, is to be measured. The differential pressure in the installation shown in FIG. 1 is developed across a conventional orifice plate 12 so that low pressure is developed in low pressure line 14 and corresponding higher pressure is developed in high pressure line 16. In FIG. 1, the process fluid is assumed to flow from left to right. Line 14 and 16 are connected to a differential pressure transmitter 18 shown in cross-sectional view. The output of transmitter 18 is an electrical signal which may be utilized in many ways, for example, to indicate process fluid flow rate by means of meter 21. Pressure in high pressure line 16 is applied to chamber 20 of transmitter 18 within body 22. The pressure in chamber 20 acts against isolation diaphragm 24 which may be constructed from any conventional material of a sufficiently high flexibility so that the spring rate thereof does not adversely affect the operation of the device. Isolation diaphragm 24 operates a relatively incompressible fill fluid, not shown, within chamber 26. The fluid in chamber 26 acts against sensing diaphragm 28 which is constructed in any conventional manner so that within the operating range of pressures applied to the transmitter its spring rate is linear. The periphery of sensing diaphragm 28, which may be circular, is firmly secured by any conventional mounting means to body 22 as shown. Affixed to the center of sensing diaphragm 28 is central hub 30 which acts against axial hub 32 affixed to the free end of spring member 34. The opposite end of spring member 34 is affixed to body 22 at point 36. Axial hub 32 carries an axial member, such as threaded shaft 38. Movable core or armature 40 is affixed to shaft 38 adjacent the free end thereof and cooperates with stationary cores 42 to form a position transducer. Appropriate electrical means form a signal indicating the relative positions between movable core 40 and stationary cores 42 which signal is indicated in indicator 21.
In a typical pressure transducer according to the prior art, as shown in greater detail in FIGS. 2 and 3, the body 22 is normally of stainless steel. Body 22 contains the chambers 26 and 29 separated by sensing diaphragm 28. Body 22 has a passage 44 communicating with an outer face 46. Isolation diaphragm 24 is disposed over the outer face 46 of body 22. The space between isolation diaphragm 24 and outer face 46 as well as passage 44 and chamber 26 up to sensing diaphragm 28 are filled with a fill fluid 48 such as silicone oil. A pressure body 50 is disposed adjacent isolation diaphragm 24 as shown. Pressure body 50 in conjunction with isolation diaphragm 24 forms chamber 20 having a process connection passage 52 communicating therewith. Process fluid (as from high pressure line 16) to be measured for pressure is introduced into chamber 20 through passage 52 to press on isolation diaphragm 24. To prevent the leakage of fluid 48 after it is in place, isolation diaphragm 24 must be hermetically sealed to body 22. In the standard prior art pressure transducer of FIGS. 2 and 3, this is accomplished by providing a weld ring 54 as shown disposed between isolation diaphragm 24 and pressure body 50 substantially over the entire interface thereof. A weld 56 is then created to bond body 22, isolation diaphragm 24, and weld ring 54 together around the outer periphery of the pressure transducer. An elastomeric seal 58 is then provided between pressure body 50 and weld ring 54 to prevent the leakage of the process fluid from chamber 20 when introduced therein under pressure.
In applications wherein it is desired to make a pressure reading on corrosive fluids, the construction of the prior art, as shown in FIGS. 2 and 3, quickly fails. Typically, in order to provide a response free from influence of the spring rate of the diaphragm, the diaphragms are made quite thin. Consequently, corrosive fluids quickly destroy portions or all of the isolation diaphragm. In many of such applications, it is possible to reduce corrosion of the isolation diaphragm by employing a tantalum diaphragm. However, a method of successfully constructing a pressure transducer employing such a tantalum isolation diaphragm has heretofore evaded accomplishment. Typically, standrd isolation diaphragm and weld ring materials are 316 stainless steel, Monel, and Hasteloy C. All of these materials melt at approximately 2500.degree. F. and are easily welded to a stainless steel body. Tantalum, however, melts at 5425.degree. F. and has a temperature coefficient only about one half that of stainless steel. It is not easily welded to stainless steel parts without cracking and/or formation of brittle tantalum hydrides. The extremely thin tantalum isolation diaphragm required further adds to the problems.
Wherefore, it is the object of the present invention to provide an improved pressure transducer and method of manufacture employing a tantalum isolation diaphragm which can be hermetically sealed to the stainless steel body of the transducer.